With the burgeoning expansion of the information technology and the high-tech industry, uninterruptible power supply (or UPS) has been widely used as an emergent power supply device for a variety of electronic devices. Most of the sophisticated electronic instruments and communication equipment rely on high-quality power supply to maintain normal operations. Contemporarily, UPS has served as an optimum solution to ensure the best power supply quality under the blackout condition.
FIG. 1 is a systematic block diagram partially showing the construction of an uninterruptible power supply, in which the uninterruptible power supply is operating under the condition that the commercial power source is abnormal or interrupted. When the commercial power source is abnormal or interrupted, the DC/DC converter 21 is configured to convert the DC voltage provided by the battery pack 23 into a boosted DC voltage, and the inverter 22 is configured to convert the boosted DC voltage into an output AC voltage for use by a load (not shown). In general, the output AC voltage is a square-wave AC voltage, which would cause damages to an inductive load, such as transformer or electromotor. Therefore, a high-level uninterruptible power supply usually has to provide a sinusoidal-wave output AC voltage to ensure the normal operation for a load when the commercial power supply is abnormal or interrupted. However, if it is desired to enable an uninterruptible power supply to output a sinusoidal-wave output AC voltage, the uninterruptible power supply has to be designed with a complicated circuit construction, which implies the increase of the cost of the uninterruptible power supply. In the meantime, complicated circuit construction would aggravate the power loss of the uninterruptible power supply.
It is therefore a tendency to improve the circuit design of the conventional uninterruptible power supply to enable the uninterruptible power supply to provide a sinusoidal-wave output AC voltage with a simplified circuit construction.